1. Field of the Invention
The present disclosure relates to an image heating device for heating an image formed on a recording medium and, in particular, to an image heating device usable as a fixing device or a gloss providing device mounted on an image forming apparatus, such as a copying machine or a printer, which uses an electrophotographic system.
2. Description of the Related Art
In recent years, a color image forming apparatus has appeared. Japanese Patent No. 4474478 discusses a system which provides a heating belt with an elastic layer as a fixing device used in an electrophotographic color image forming apparatus. The reason the heating belt provided with the elastic layer is used is that the surface of the heating belt is caused to follow the unevenness of a toner layer to uniformly melt the toner layer. A resin or metal is used as a base layer of the belt according to applications.
Some image heating devices using a heating belt can control the electric potential of the surface of the heating belt to prevent electrostatic offset or an image defect (hereinafter referred to as tailing) caused by toner on the surface of a recording material blown away by water vapor generated by the recording material. For example, there is sometimes a case where a contact is provided on the end of the heating belt to perform ground (GND) connection or bias application. The surface of the heating belt is controlled to have an electric potential with the same polarity as that of the toner to generate electrostatic force pressing the toner on the surface of the recording material against the recording material, thus preventing the toner from being moved and an image from being deteriorated thereby.
There may be two methods for providing the heating belt with a contact: one for causing a conducting member to contact the inner surface of the heating belt; and the other for applying a conductive brush to an exposed portion of the elastic layer, which is partly peeled away and exposed, of the heating belt. However, if the conducting member is provided on the inner surface of the heating belt, a space inside the cylinder of the belt is decreased along with the decrease of the diameter of the heating belt, thus making it difficult to arrange components and to ensure a distance for insulation between the belt and internal components. If apart of the brush falls out, or the conductive component is worn out, it may remain as a foreign matter in the heating belt to increase drive torque. For that reason, it is desirable to ensure conduction such that the elastic layer of the heating belt is partly peeled away and the conductive brush is applied to the exposed portion from the outside of the belt (refer to Japanese Patent No. 4054488). However, the heating belt may deviate to the left or the right along with the rotation thereof. In this case, the end of the heating belt contacts an edge regulation portion such as a side plate or a flange of the image heating device and the heating belt is rotated while sliding on the edge regulation portion. If the heating belt shifts to the side where the end of the heating belt has the elastic layer, the elastic layer in addition to the base layer contributes to the strength of the heating belt, so that breakdown hardly occurs from the end. On the other hand, if the heating belt shifts to the side where the base layer is exposed, since nothing protects the base layer and the base layer is very thin metallic layer, fatigue breaking is liable to occur. The breakdown of the end of the heating belt may terminate the lifetime of the image heating device before the lifetime of the image forming apparatus is reached, so that inefficiency occurs that requires repair or replacement of the image heating device.